The present application relates to co-pending U.S. pat. application, Ser. No. 09/884,702, filed on Jun. 19, 2001, titled xe2x80x9cPiezoelectric Actuated Optical Switch,xe2x80x9d which claims the priority of U.S. provisional patent application, Ser. No. 60/246,284, filed on Nov. 6, 2000, both of which applications are assigned to the same assignee as the present application, and are incorporated herein by reference.
The present invention relates generally to optical signal switching, and particularly to a piezoelectric actuated device for switching an optical signal. More specifically, the present invention relates to an active optical modulator that allows switching from a reflecting state to an anti-reflecting state and vice versa. The switch is based on the precise controlling of an air gap between a thin film membrane and a substrate. The thin film membrane is deformed by a miniaturized adaptive material, such as electrostrictive or piezoelectric (PZT) material. Maximum optical reflection is realized when the air gap is equal to a quarter wavelength of the optical beam, while anti-reflection is achieved when the thickness of the air gap is different from the quarter wavelength.
With the increasing popularity of the World Wide Web (xe2x80x9cthe webxe2x80x9d), there is a continual need to increase the available communication bandwidth. The constant traffic on the web requires an infrastructure that is dynamic to accommodate new needs as they emerge. One of the most pressing challenges is the underlying pipeline, that is the bandwidth which accommodates new users and applications. Some of these applications include as video on demand, video conferencing, and so forth.
A number of photonic solutions have been proposed to increase the available network bandwidth. These solutions range from point to point connections to wavelength division multiplexed passive optical network systems. The latter solution is effective in principle, however the cost associated with photonic devices in these systems has been an impediment to their acceptance and rapid deployment.
Optical data transmission offers many advantages over electrical and broadcast transmission. However, switching optical data from one channel to another has proven to be problematic. Fundamentally, a beam of light is unaffected by passage through an electric or magnetic gradient, thus the usual solid-state methods for switching electric signals are not effective to switch optical signals. Accordingly, various mechanical techniques relying typically on reflection or refraction have been developed to divert optical signals.
FIG. 1 is a schematic diagram of a conventional optical switching array 10. The switching array 10 includes a plurality of input ports, i.e., 12, 16, and output ports 14 arranged in columns and rows. To switch an optical signal from a first input port 16 to the output port 14, a diverter 18 located at a point of intersection between the axes of the two ports 16 and 14, diverts the beam from the input port 16 to the output port 14. The diverter 18 can be a mirror, a light pipe, a refractive medium, or the like. Most diverters 18 require a form of actuation to move them into or out of the path of a light beam.
FIG. 2 shows a cross-section of a MEMS diverter 18. The diverter 18 is comprised of a base 32 suspended within a frame 34. The base 32 includes a reflective coating 36. Between the frame 34 and the bottom of the base 32 is an interdigitated electrostatic actuator 37 comprising interdigitated fingers 38 and 39 of the base 32 and frame 34, respectively. The interdigitated electrostatic actuator 37 is actuated by applying electric charges to surfaces of fingers 38 and 39 to cause them to attract each other. The electric charges can be applied to specific fingers 38 and 39, or to sets of fingers 38 and 39, to modify how much force is applied, and in what direction, to control the induced tilting of base 32.
Conventional MEMS diverters, however, suffer from some drawbacks. In addition to being expensive to produce, they are also sensitive to electrostatic discharges (ESD) and microcontamination. It will be readily appreciated that ESD can destroy the interdigitated electrostatic actuator 37 by melting or fusing fingers 38 and 39. Similarly, microcontamination in the form of fine particles or surface films, for example, can mechanically jam the interdigitated electrostatic actuator 37 and prevent it from actuating. Microcontamination can also create an electrical short between fingers 38 and 39, thereby preventing actuation.
A low-cost silicon optical modulator based on micro electro mechanical systems principles (MEMS) has been proposed, offering a low-cost, high production volume modulator. This device has been designated MARS, which is an acronym for Moving Anti-Reflection Switch. In one form, this device has a multi-layer film stack of polysilicon/silicon nitride/polysilicon, wherein the polysilicon is doped and forms the electrode material. A precisely controlled air gap between the film stack and the substrate allows switching from a reflecting state to an anti-reflecting state.
The operating principle of a conventional MARS device 100 is illustrated in FIGS. 3, 4, and 5, and is based upon the change in an air gap 105 between a suspended membrane 110, e.g., a silicon nitride film, and an underlying substrate 120. The membrane 110 has a refractive index equal to the square root of the refractive index of the substrate, and a thickness equal to xc2xc the wavelength (xcex/4) of an incident light beam.
If the membrane 110 is suspended above the substrate 120 such that when the air gap 105 equals xcex/4, a high reflection state is achieved, otherwise, including when the air gap 105 is close to zero, an anti-reflection state is achieved. These states also hold true for any value of mxcex/4, wherein an even number m represents an anti-reflecting state (or mode), and an odd number m represents a reflecting state. An exemplary MARS structure that is referred to as a double-poly MARS device, is described in U.S. Pat. No. 5,654,819.
To activate this MARS device, two electrodes are provided and positioned on top of the membrane 110 and the substrate 120, with a voltage selectively applied therebetween. The applied voltage creates an electrostatic force that pulls the membrane 110 physically closer to the substrate 120. When thickness (depth) of the air gap 105 between the membrane 110 and the substrate 120 is reduced to substantially xcex/2, an anti-reflective device exhibiting substantially zero reflectivity is produced.
While this MARS device 100 provides certain advantages over other prior conventional devices, it has a potential catastrophic failure mode due to the lower polysilicon metallization. This failure mode is illustrated in FIG. 5, where in certain adverse conditions, such as large changes in the dielectric properties of the air gap 105, or with unusual voltage surges (i.e., electrostatic discharge or ESD) in the switching signal the membrane 110 undergoes excessive deflection, and shorts to the substrate 120, resulting in a device (100) failure.
Accordingly, it would be desirable to have an optical switching device that can redirect a beam of light that is less susceptible to microcontamination and ESD failures, and that is readily fabricated according to developed microfabrication technologies.
The present invention addresses and resolves the foregoing concerns that could lead to potential failure of the MEMS-based devices, namely (i) spurious voltage spikes and (ii) large changes in the dielectric properties of the air in the air gap.
The active optical switch of the present invention includes a thin film membrane, that is suspended over a substrate, and that is mechanically deformed by a miniaturized motor, to perform the reflection and anti-reflection switching. In a preferred embodiment, the motor is comprised, for example, of an adaptive or electrostrictive material, such as piezoelectric (PZT). The displacing voltage is applied to the motor rather than to the membrane.
Consequently, the membrane and the substrate are not electrically charged as are the corresponding components of the conventional MARS device described above in connection with FIGS. 3 through 5. Thus, the switch of the present invention is tolerant of a direct contact between the membrane and the substrate, thereby solving the spurious voltage spikes concern.
In addition, in further contrast to the conventional MARS device described above, the switch of the present invention neither uses nor relies on the air properties in the gap between the membrane and the substrate as an electrically conductive medium to activate the motion of the membrane. The movement of the membrane is caused by the contraction or expansion of the motor.
This novel design addresses and solves the concern facing the MARS device described above, namely large changes in the dielectric properties of the air in the air gap. Moreover, due to fact that the substrate is no longer required to be electrically charged, it does not have to be made from special material, such as silicon, nor fabricated using special microfabrication techniques, in effect reducing the cost, labor, and material of the optical switch.